1. Field of the Invention
This invention relates to cardiac pacers and more particularly to a rate adaptive pacer which alters the pacing escape interval in response to the physiologic demand of the patient.
2. Description of the Prior Art
Implantable medical devices for the therapeutic stimulation of the heart are well known in the art. Initially these cardiac pacers were asynchronous in operation providing stimulating pulses to the heart at a fixed rate independent of the physiologic demand of the patient.
Subsequently, demand pacemakers were developed as exemplified by U.S. Pat. No. 3,478,746 to Greatbatch. These devices provide stimulating pulses to the heart only in the absence of naturally occurring cardiac activity. This form of pacer permits the patient's underlying cardiac rhythm to inhibit the pacemaker if the patient's intrinsic heart rate is above the preset escape interval of the pacer. However, if the patient's intrinsic cardiac activity drops below the minimum rate set by the escape interval of the pacer, stimulating pulses will be supplied to the heart. In this fashion the demand pacemaker provides a lower boundary rate below which the patient's heart will not be permitted to drop. The therapeutic benefit of such demand pacemakers was enhanced by the development of the hysteresis type pacer known from U.S. Pat. No. RE. 28,003 to Gobeli, which provides two escape intervals.
The hysteresis pacemaker permits the heart to inhibit the pacemaker down to a sentinel rate set by the hysteresis pacemaker. However, if no intrinsic cardiac activity is detected during the sentinel escape interval, the patient's heart will be stimulated at a nominal escape interval which is somewhat shorter than the lower hysteresis rate. In operation, the hysteresis pacer alters the escape interval in response to detected cardiac events.
More recently, pacers have been disclosed which rely upon a historical average of detected cardiac activity to set the escape interval. An example of one such pacer is taught by U.S. Pat. No. 3,921,642 to Preston.
Other forms of rate adaptive pacers have also been proposed. These pacers rely on the sensing of atrial activity, blood PH, respiratory rate and QT interval data to alter the pacer's escape interval. Discussions of some of these prior art proposals may be found in Relation Between the QT Interval and Heart Rate, Rickards and Norman, Britt Heart J., 1981; 45; 56-61 and A Physiologically Controlled Cardiac Pacemaker, Krasner, Voukydis and Nardella, J.A.A.M.I., Volume I, No. 3, 1966; 14-20.
This historical progression indicates the desire to provide a pacer which alters the escape interval in response to the physiologic demand or needs of the patient.
The structure of the present invention includes a force sensor located within the pacer itself. A prior art example of a related structure is known from U.S. Pat. No. 3,777,762 to Nielsen.